Trim layer won't trim to playhead

Compositing and 3D apps almost always trim or set the out point to the start of the last frame so you can see what is going on in the last frame. This makes sense so you know where to put the next element in the composition. You always want to see the current frame when you are trying to line things up. If the CTI parked at the end of the current frame, when you added something it would be out of sync by one frame.

Editing apps always trim/cut to the end of the current frame. This makes more sense for editing. After Effects is not a Video Editing app. It is designed to create shots, and you just have to adjust to seeing the last frame when you set an outpoint.  The CTI is always going to park at the start of the current frame.

If you split a layer (Shift + Ctrl/Cmnd + d) the original layer will be cut in the same way a video editor cuts. The cut will be at the start of the current frame, so the old layer will end at the CTI (Current time indicator), and the new one will start there. Splitting a layer is making a cut. Going to a specific frame always shows you the frame. 

I hope that makes sense. I admit it's a bit of a newsence if you are trying to use After Effects to edit a movie, but editing a movie in AE is a pretty bad idea anyway so pros don't do it.